Originally Posted by Big Pistons
A TCAS RA only works properly if both aircraft have an operational TCAS.
What do you mean, "properly"? The only advantage of having TCAS in each is that they talk to each other eg I'll go up, you go down". The system still works fine with only one TCAS.
What about all those lighter types that have a transponder but no TCAS, mixing it with airliners in the terminal areas?